This invention relates to the pulping of waste paper products for recovery of reusable paper making fibers therefrom, and especially to improvement of such pulping operations from the standpoints of both efficiency and high quality of yield.
A problem of increasing magnitude in the pulping of waste paper products has been the steady increase in the amount and nature of the contaminants mixed therewith in commercially obtainable waste paper, the contaminants now commonly averaging of the order of 15% by weight. Of particular importance is the amount of lightweight contaminant junk, primarily in the form of plastic products of many kinds and especially plastic sheet and film and also pieces of plastic foam.
In the past, many of the common contaminants of waste paper could be eliminated from the pulper tub by the use of a junk remover, a typical example being shown in Baxter U.S. Pat. No. 3,549,092. Such a junk remover relies on gravity discharge through a downward chute from the pulper tub of iron and other junk material of substantially higher specific gravity than paper fibers. But such junk removers have proved to be ineffective for removing lightweight junk, for two principal reasons.
One reason is the obvious one that material lighter than water will not readily flow down the chute which connects a pulper tub with its junk remover. The other is that the normal operation of a pulper rotor tends to force sufficient liquid from the tub to the junk remover when the pulping operation commences to maintain a higher static head in the junk remover than in the tub, commonly of the order of two or more feet. Further, the common practice is to add fresh liquid to the tub by way of the junk remover, in order to wash fiber back into the tub from the high specific gravity pieces traveling through the chute from the tub, and this increases the opposition to the flow of light materials from the tub.
The result of these conditions is that when a waste paper pulper -- whether or not it is equipped with a junk remover -- is operated on a continuous basis, with continuous extraction, through a perforate extraction plate, of a slurry of sufficiently small particle size and continuous replacement of water and furnish, plastic tends to accumulate in the tub until the amount of extracted fiber drops below an acceptable rate, a condition which the industry calls "constipated". It is then necessary to discontinue pulping and empty the accumulated junk manually from the tub.
The development of this condition has three significant disadvantages. Running of the pulper until the paper fiber can no longer be extracted not only results in loss of production of recovered paper fiber but also produces increased and unnecessary wear on the pulper rotor and its extraction plate. In addition it results in extraction of a substantial amount of small plastic particles with the paper fiber, as the quantity of plastic in the tub increases to the point where it comes into contact with the rotor, and such small pieces of plastic are difficult to separate from the paper fiber, especially if the holes in the extraction plate are small. At the same time, manual emptying of accumulated plastic is expensive and time consuming, and it also results in the loss of a substantial amount of fiber which remains commingled with the plastic and is therefore eliminated along with the plastic.
U.S. Pat. No. 4,030,671 issued June 21, 1977 to Joseph Walter Couture taught that these disadvantages of past practice can be overcome, and the effectiveness of a junk remover greatly improved, by maintaining the liquid level in the junk remover lower than in the pulper tub and thereby inducing liquid flow from the tub into the junk remover. In accordance with that patent, this is done by connecting the inlet of a pump to the junk remover casing at a level below the minimum operating level in the tub, and withdrawing liquid from the junk remover and recirculating it back to the tub under controlled conditions establishing the desired lower liquid level in the junk remover than in the tub, e.g. lower by about a few inches.
The effect of this removal of the normal static head conditions is first to induce flow through the chute from the tub into the junk remover. Lightweight trash circulating in the tub will be entrained in that flow, and as soon as it enters the junk remover, it will rise to the top and thus be trapped against return to the tub. The resulting accumulation of lightweight trash at the top of the liquid in the junk remover is lifted out for removal by the perforated conveyor buckets which are standard equipment in a junk remover.
The system of the Couture patent has proved to be so effective in one commercial installation, handling commercial waste paper containing approximately 15% trash, that the pulper operated continously at practical extraction and horsepower rates when the recirculating pump was operated for only two 15-minute periods during each 8-hour shift. In this same installation, prior to application of the invention thereto, the junk remover had been so ineffective in eliminating lightweight trash that its use for that purpose had been discontinued.
The system of the Couture patent, however, has proved to be almost too effective in another installation, in the sense that the plastic sheet material accumulated so rapidly in the collection box into which conveyor buckets dumped that its removal from the collection box became a practical problem. In addition, the rapidly accumulating lightweight trash included or carried with it an undesirably large proportion of good fiber and still undefibered pieces of recoverable paper material. There is also a possibility in the Couture system that the inlet of the recirculating pump may be plugged by very large pieces of junk or undefibered waste paper or board. The present invention was developed to take care of such situations, as described hereinafter.